Crash Tests: What You Need to Know

Overall, passenger vehicles are safer than they've ever been, but the crashworthiness of individual models varies greatly, even within a vehicle class. New roof-strength tests also reveal that one version of a model can fare better than another. Because of this, understanding how a model is expected to perform in a crash is important before buying your next car.

Not All Results Can Be Compared

A Ford Mustang is put through an IIHS crash test.

Model-to-model comparisons of frontal crash ratings are valid only within a vehicle class or between models of comparable weight (as long as they're within 250 pounds of each other). The test reflects how the vehicle would fare in a collision with another of the same model, not versus a larger or smaller vehicle (or a lower- or higher-riding vehicle). A heavier vehicle protects its occupants better than a lighter one if all other factors are equal, but they almost never are. So a large vehicle with a Poor rating is not necessarily safer than a small vehicle with a Good rating. Unfortunately, researchers have not yet devised a reliable method for reporting the effect of size differences on a vehicle's score.

Likewise, the Insurance Institute for Highway Safety's rear-crash head-restraint ratings consistently test how well a stationary seat protects against whiplash by simulating a 20-mph rear crash. IIHS combines the results with an evaluation of the seat's geometry to arrive at a rating. The agency requires a Good rating in the rear test in order for a model to earn the group's Top Safety Pick designation. Manufacturers were able to change their seats in model-year updates, resulting in many 2009 Top Safety Picks. For 2010, in order to get a Top Safety Pick, a vehicle must get Good scores in the roof-strength test (See Roof-Strength Tests Provide Key Rollover-Protection Data). Because of that, there are fewer 2010 Top Safety Picks in each category. Back to top

Crash Tests Differ by Agency

There are two testing agencies and they perform different types of frontal tests. The National Highway Traffic Safety Administration crashes cars head-on into a solid immovable barrier. Neither the angle nor the obstacle corresponds with the majority of real collisions. IIHS conducts a frontal-offset crash into a deformable barrier that reacts like another vehicle. This gauges how well half of the vehicle's front end absorbs crash energy. Many experts say this test is more revealing and better represents the majority of real-world crashes. Additionally, NHTSA has acknowledged that its New Car Assessment Program hasn't evolved quickly enough to account for marketwide improvements in vehicle crashworthiness. Simply put, too many models are getting high scores from NHTSA, and the differences among them — which exist — aren't reflected in the ratings. NHTSA announced in 2008 that updates would first appear in 2010-model-year crash tests, but postponed the changes for the 2011 model year. Back to top

Side-Impact Ratings Have Deficiencies

Side-impact crash-test results currently aren't as abundant. Though NHTSA has tested more models than IIHS for side-impact protection, these tests are inadequate for two reasons:

The sled employed to "T-bone" the stationary test vehicle has the height and mass of a car, not an SUV or a pickup truck. This tends to minimize its intrusion into the cabin — making it a best-case scenario.

NHTSA's chance-of-injury data are based on trauma to the test dummies' torsos, not their heads. Experience has shown that occupants' heads are more susceptible to injury in a side impact, and head injuries are more often serious and potentially fatal, according to experts.

A Mercedes-Benz E-Class undergoes an IIHS side-impact crash test.

NHTSA calls out a "safety concern" on some ratings on its website, but they don't affect the car's star rating. For example, the 2007 Chevrolet Cobalt two-door report stated: "Safety Concern: During the side-impact test, the head of the driver dummy struck the windowsill, causing a high head acceleration. Head impact events resulting in high accelerations have a higher likelihood of serious head trauma." Yet the car received a respectable double-four-star side-impact rating.

When tested with optional side curtain airbags, the 2007 Cobalt two-door report raised no extra safety concerns, but the Cobalt got a three-star side-impact rating. (Retested for 2008 with side curtains, now standard, the Cobalt scored four stars for side impact, and no safety concern was listed.) NHTSA will begin to factor head injury into its bottom-line results, but not until the 2011 model year. The IIHS side-impact test measures head injury and employs a sled as high and heavy as a full-size SUV or pickup, creating a more dangerous scenario. Unfortunately, IIHS began this program only recently, so ratings go back only a few model years. Because the size of sled is consistent, comparisons of side-impact ratings are valid across vehicle classes.

It's important to scrutinize crash-test reports — not just to determine if the car has side airbags, but to know if they are standard or optional on the car you're considering. In some cars, side bags have meant the difference between getting a top or a bottom score, and it's up to you to make sure the car you buy has them. Side-impact tests use properly positioned, belted test dummies, which doesn't tell us what would happen if an occupant were out of position — in which case the side airbag firing can itself be hazardous, especially for children.

To address this, NHTSA reports now have an "SAB Out Of Position Testing" field that may read "Meets specifications." You should know that this result is being reported by the automaker after voluntary testing — it's not a test performed by NHTSA. People concerned about injuries from side airbags should avoid the seat- or door-mounted type, or buy a car that disables those airbags when the occupant is out of position. Honda and Acura have pioneered this feature. Curtain airbags are considered to be less dangerous. Back to top

Government Rollover Ratings Have Shortcomings

Auto manufacturers and safety experts considered NHTSA's original Rollover Resistance Ratings, begun in the 2001 model year, inadequate at judging a model's rollover propensity because they were based on a mathematical calculation of the vehicle's center of gravity. Starting with the 2004 model year, NHTSA combined this calculation with a "fishhook" dynamic driving test in which the test vehicle swerves suddenly, then overcorrects. The combined results, NHTSA Rollover Ratings, give a percentage chance of rollover — a star rating based on this chance and whether or not the model tipped up on two wheels during the fishhook test. While many see this as a step in the right direction, some automakers still criticize NHTSA for extrapolating some conclusions. Back to top

Roof-Strength Tests Provide Key Rollover-Protection Data

Where NHTSA attempts to relate a model's propensity to roll over, new roof-strength tests from the IIHS reflect how the roof might protect occupants when a rollover occurs. Using its familiar scale — Good/Acceptable/Marginal/Poor — IIHS has begun to rate 2010 models based on how well they resist up to four times their weight in a crush test. Because weight varies among different versions of the same model, it's conceivable that two-wheel- and four-wheel-drive versions of the same model could earn different scores. The extra weight of hybrid hardware has earned the hybrid version of Ford's Escape a Poor roof-strength rating even though the non-hybrid is rated Marginal. For more information on roof strength as it relates to safety, see the related Roof-Strength Ratings Offer Insight on Rollover Safety. Back to top

Some Models Are Not Rated

If the model you seek is missing crash-test results, they may be pending or the vehicle may not be tested. Both agencies concentrate on the highest-volume vehicles. Convertibles are rarely tested for this reason, though for the first time in 2007 IIHS tested 10 models, including several best-sellers like the Chrysler Sebring and Ford Mustang. Results for new or recently reengineered models are likely to appear months after the car goes on sale because both agencies purchase their test subjects from dealerships. NHTSA notes if a vehicle is TBT (to be tested) or if results are pending or under review. IIHS has begun to offer more detailed information about whether or not test results are pending. Back to top